Cast coating process



July 11, 1967 B. s. HALSEY ETAL CAST COATING PROCESS Filed Oct. 29, 1963\EEE maomom m. mm.

/5mm 9:55 352 W835 United States Patent 3,330,688 CAST COATING PROCESSBrenton S. Halsey and Robert C. Williams, Richmond,

Va., assignors to 'Albemarle Paper Company, a corporation of VirginiaFiled Oct. 29, 1963, Ser. No. 319,768 9 Claims. (Cl. 117-64) Thisinvention relates to an improved method and apparatus for cast coating acontinuous web. More specifically, this invention relates to an improvedmethod and apparatus for cast coating a paper web when employing aliquid coating composition which is cast at temperatures exceeding theboiling temperature of the liquid contained in the coating.

The method of cast coating which comprises heating a web to atemperature of around 212 F. or more is generally referred to aspressure-cast coating since sufficient pressure must be applied to theweb during its heating to prevent the web from being separated or blownaway from the dryer drum before its coating is dried or cast, otherwisean imperfect coating will result. In conducting that process, it isdesirable to maintain an excess or puddle of the liquid coating materialat the entrance nip which is that area defined by the paper web as itcomes into contact with the casting surface. While not absolutelyessential to achieve high quality cast coatings, this technique ofoperating serves as an obvious indicator to the machine operator that heis maintaining a suitable coating consistency in relation to otheroperating parameters.

To increase production rates, the above described type of castingoperation can be readily speeded up by increasing the temperature andspeed of rotation of the dryer drum upon which casting is effected, viz.increasing the temperature above 212 F. In this manner, the wholeoperation is speeded up and the web being coated naturally moves throughthe casting unit at a greater linear speed. It has been found when castcoating a web by the above described procedure that as the temperatureof the dryer drum is increased considerably above 212 F. (e.g. 280 F.),maintenance of the proper puddle of coating material in the entrance nipby the machine operator becomes more and more diflicult inasmuch as theliquid in the coating material begins to boil away. This not onlyresults in the coating becoming more viscous which at times will causesticking of the coating to the drum, but additionally causes spatteringof the coating. Spattering is undesirable since the distinct anddiscrete particles of coating material upon contacting the castingsurface of the heated drum coalesce and become dried. These discreteparticles of dried coating material upon subsequent contact with thepuddle and the wet coated web are not absorbed thereby but rather remainseparate and distinct and at times will cause additional coatingmaterial to be pulled away from the web and cohere to the castingsurface. This results in an imperfect cast coated web having a mottledand distorted surface.

While the above problem can be overcome by having the machine operatoradjust the coating viscosity in proportion to any changes in thetemperature of the dryer drum, there are intervening factors that mustalso be correlated simultaneously by the machine operator. The netresult is that this extent of operating complexity introduces a widemargin of human error which has proven difficult to control. It can beappreciated that failure to maintain this required degree of controlresults in high waste and loss of operating time in order to cut backthe production rate and establish exactly the proper or right operatingconditions.

An object of this invention is to eliminate this close Patented July 11,1967 degree of operational control. Specifically, an object of thisinvention is to provide an improved pressure-cast coating method andapparatus whereby a puddle of coating material can be maintained at theentrance nip in a simplified manner without necessitating an extremelyclose degree of operational control, especially when pressure-castcoating at high pressures and temperatures. These and further objectswill come to light as the discussion proceeds.

The drawing depicts a schematic elevational view of the apparatus ofthis invention wherein the process of this invention can be practiced.

The above objects are accomplished pursuant to the practice of thisinvention which provides a method of coating a web with a mineralcoating which method comprises: passing the web through an entrance nipbetween a pair of turning rollers, one of said rollers being main:tained at a temperature at least equivalent to the boiling temperatureof the liquid in a wet thermosettable mineral coating; simultaneouslypassing a cooled endless metal belt through said nip; and maintaining apuddle of said wet thermosettable mineral coating within the nip definedby said belt and said web. In conducting the above process, it ispreferred to employ a metal belt having an extremely smooth surface,especially a chromium plated extremely smooth surfaced flexible metalbelt. Moreover, the improved method of cast coating made possible by wayof this invention is especially applicable to pressurecast coating withvapor receiving means in combination with pressure means for urging thepaper web against the dryer drum which can be heated to temperatureswell above 212 F., e.g. on the magnitude of 350 F. When pressure-castcoating in this manner, it is preferred to heat the dryer drum 18 to atemperature within the range of from about 220 F. to about 350 F.,especially at about 240 F. to about 280 F. Moreover, it is preferred toexert a pressure upon the web by pressure means 20 within the range offrom about 5 p.s.i. to about 50 p.s.i., particularly from about 10p.s.i. to about 35 p.s.i. By this embodiment and operating under theseconditions, cast coated paper having optimum properties is produced atlowest cost The basic apparatus constituting a part of this invention isdepicted in the drawing which further illustrates the preferred type ofcast coating operation to which the present invention is especiallyadaptable. Referring to the drawing, the improved cast coating means ofthe instant invention comprises the rotatable drum means 18 which meansfurther comprise heating means 35, the latter preferably comprisingsteam injection and condensate discharge means. A system of rollermeans, for example the rollers 15 and 17, is provided for conveying theendless metal belt 12 around a portion of the surface of the drum 18,the outer surface of the belt 12 being adapted to serve as a drying andcasting surface. The web conveying means 21 is positioned and adapted(a) to bring a wet coated web 10 into contact with the outer surface ofthe belt 12 at least by the point at which the belt first meets the drum18, (b) to define at the point of said contact a nip capable ofreceiving a puddle 11 of the coating composition, and (c) to maintaincontact beyond the point at which the belt 12 first meets the drum 18.In combination with the metal belt 12, which as brought out above ispreferably a chromium plated extremely smooth surface flexible metalbelt, is cooling means 13 which is preferably water maintained at aboutroom temperature within container means 14.

Thus it can now be appreciated that among the advantages and features ofthe present invention is that the belt 12 keeps the puddle 11 away fromthe hot drum 18 and thus minimizes the amount of heat picked up by thepuddle of coating material 11. This coupled with the fact that the belt12 is additionally cooled by cooling means 13 makes it possible tooperate with a very liquid flooded nip 11 while avoiding spattering ofthe coating composition. As indicated above, spattering of the coatingcomposition at the entrance is extremely undesirable inasmuch as thedistinct particles of coating materials will coalesce and adhere to thecasting surface and upon subsequent contact with the wet coated webundergoing drying, will not blend in and form an integral part of thecoating but rather will remain a separate entity the configuration ofwhich is assumed and reflected in the cast coating, viz. an imperfectcast coated web having mottled surface. Even if spattering should occurin the practice of this invention, the above effect is minimized sincethe distinct particles of coating material do not contact the castingsurface when it is hot. Consequently, the particles of coating materialare not dried upon the casting surface and hence absorbed by the puddlemaintained in the entrance nip. Additional advantageous features of theinstant invention is that the metal belt 12 serves as a casting surfaceunlike the conventional smooth surface dryer drum as presently employedin the art. This minimizes the cost of operation inasmuch as in theevent of damage to the casting surface, the metal belt need only bereplaced in lieu of the rather expensive dryer drum. The amount of downtime is also minimized since the metal belt can be more quickly replacedthan the dryer drum. Moreover, employing the same production line, it ispossible to produce economically coated products having various degreesof surface smoothness or embossing thereon by merely substituting aflexible metal belt having the desired surface finish. Otherwise, it isnecessary to change the dryer drum, a practice which is obviouslycumbersome, or at least have different production lines which isobviously more expensive.

As brought out above, the drawing depicts the preferred method and meanswherein the practice of this invention can be conducted. The wet coatedpaper stock as fed to the casting unit has thereon a coatingcomposition-which is applied at about 8 to about 25 pounds per 3,000square feet of base stock. The coating contains a heat-vaporizableliquid, e.g. water as employed in conventional pigmented coatings, e.g.clay, titanium, dioxide, and the like based coatings which furthercomprise an adhesive, e.g. starch, casein, and the like. It isunderstood of course that the coating composition can contain othermaterials, for example waterproofing agents, defoamers, wetting agents,release agents, and the like. In fact, an ancillary feature of thepresent invention is that the loss of volatile additives by vaporizationoccurring at the flooded nip is similarly greatly reduced or eliminatedby operating in this manner. In terms of our preferred embodiment, thewet coated paper 10 is fed to the casting unit comprising the chromiumplated smooth surfaced fiexible metal belt 12 in combination with thedryer drum 18 and heating means 35. The wet coated paper web 10 is fedinto the nip defined by the position of the drum 18 and roller 21wherein the puddle 11 is maintained between the wet coated web 10 andmetal belt 12. The metal belt 12 is cooled by conveying it over roller15 into contact with a source of cooling water 13 within the containermeans 14. Wiping means 16, such as conventional papermakers felt, isprovided to remove excess water from the flexible metal belt which isthen conveyed over roller means 17 into contact with the flooded nip 11and the heated dryer drum 18. The web 10 is pressure'cast by beingcompressed by the pressure belt 20 over the area defined by the extentor degrees of Wrap of the pressure belt 20 around the casting surface 12which conforms to the shape of the dryer drum 18. The degrees of wrap ofthe casting surface area is determined by the position of the rollers 21and 24 which together with rollers 22 and 23 constitute a conveyingsystem for the pressure belt 20. Operating in combination with thepressure belt 20 is vapor receiving means 19, generally conventionalpapermakers felt. The vapor receiving means 19 receives moistureemanating from the coating formulation which is driven therefrom duringthe course of casting it upon the paper web 10. The vapor receivingmeans 19 can be separately dried if desired by such means as the heateddrum 27 which together with rollers 25, 26, and 28-32 constitute theconveying system for means 19. After casting, the cast coated paper thenleaves the heated dryer drum over the roller means 33.

By way of example, employing the apparatus arrangement depicted in thedrawing a paper web 10 comprising an uncalendered board having a densityof 7 lbs./ream/ mil and 18 mils thick as prepared from 50 percent Kraftand 40 percent Groundwood is first given a wet coating composed of partsclay, 18 parts protein, and sufficient water to make a 55 percent solidsolution which is applied to the web 10 in sufficient quantity toproduce a coating of 20 lbs/ream. The wet coated web 10 is fed into thecasting unit comprising the dryer drum 18 which is heated to atemperature of about 350 F. A puddle 11 of the coating material iseasily maintained at the entrance nip at all times. The pressure belt 20by virtue of tensioning means 34 is caused to exert a pressure of about33 p.s.i. continuously and uniformly upon the paper web 10 during itscontact with the metal belt 12. The vapor receiving means 19 comprises Ainch thick conventional papermakers felt. The temperature of the coolingwater 13 is maintained at essentially room temperature. By means of thisprocedure employing a dryer drum of 6 foot diameter, production ratesexceeding 400 feet per minute are easily attained without entailingexcessive attention on the part of the machine operator to maintain acool flooded nip or puddle 11 of the coating material. Moreover, rapidchanges in production rates can be readily accomplished, viz. from 450f.p.m. to 550 f.p.m. in a matter of minutes without adjusting theconsistency of the coating composition but still having the same numberof machine operators in attendance. Operating in this manner produces acast coated paper having an extremely smooth finish and high gloss.

What is claimed is:

1. A process for pressure cast coating a web with a thermosettablemineral coating comprising, in combination,

(a) passing said web through an entrance nip into a pressure areadefined by the contact of at least two moving endless belts one of whichis metal and one of which is a porous material,

(b) maintaining a puddle of liquid solution of unset said thermosettablemineral coating at said entrance nip between said web and the one movingendless metal belt, said porous belt contacting the uncoated websurface,

(c) heating the moving endless metal belt after contact with said puddleat said entrance nip to a temperature at least equivalent to the boilingpoint of the liquid of said liquid solution,

(d) maintaining the coated web within said pressure area for a periodsufiicient to cause said thermosettable mineral coating to becomepressure-cast on said web,

(e) separating said at least two moving endless belts to release thepressure on said web,

(f) recovering the resulting thermoset pressure castcoated web, and

(g) cooling the separated said one moving endless metal belt to atemperature at least below the boiling point of said liquid prior to itscontact with said puddle.

2. The process of claim 1 further characterized by the heating of themoving endless metal belt being accomplished by contacting the belt witha rotating heated drum.

3. The process of claim 1 further characterized by said heating being toa temperature of from about 220 F. to about 350 F. and said pressurearea between said at least two moving endless belts being sufiicient tocompress the moving web there between at a pressure of from about 5p.s.i. to about 50 psi. in excess of ambient pressure.

4. The process of claim 3 further characterized by said temperaturebeing from about 240 F. to about 280 F. and said pressure being fromabout 10 psi. to about 35 psi.

5. The process of claim 1 further characterized by said one movingendless metal belt being chromium plated at least on the side in contactwith said puddle.

6. The process of claim 1 further characterized by said cooling beingaccomplished by Water.

7. The process of claim 1 further characterized by said moving endlessmetal belt being smooth on the side in contact with said puddle.

8. The process of claim 1 further characterized by said moving endlessmetal belt having an embossing pattern on the side in contact With saidpuddle.

9. The process of claim 1 further characterized by said liquidcomprising water.

References Cited UNITED STATES PATENTS 1,944,600 1/1934 Greene 117642,442,443 6/ 1948 Swallow 117-64 2,919,205 12/ 1959 Hart 11764 2,943,9547/ 1960 Robinson et al 117-64 3,110,612 11/1963 Gottwald et a1. 11764FOREIGN PATENTS 794,460 5/ 1958 Great Britain.

MURRAY KATZ, Primary Examiner.

1. A PROCESS FOR PRESURE CAST COATING A WEB WITH A THERMOSETTABLEMINERAL COATING COMPRISING, IN COMBINATION, (B) PASSING SAID WEB THROUGHAN ENTRANCE NIP INTO A PRESSURE AREA DEFINED BY THE CONTACT OF AT LEASTTWO MOVING ENDLESS BELTS ONE OF WHICH IS METAL AND ONE OF WHICH IS APOROUS MATERIAL, (B) MAINTAINING A PUDDLE OF LIQUID SOLUTION OF UNSET,SAID THERMOSETTABLE MINERAL COATING AT SAID ENTRANCE NIP BETWEEN SAIDWEB AND THE ONE MOVING ENDLESS METAL BELT, SAID POROUS BELT CONTACTINGTHE UNCOATED WEB SURFACE, (C) HEATING THE MOVING ENDLESS METAL BELTAFTER CONTACT WITH SAID PUDDLE AT SAID ENTRANCE NIP TO A TEMPERATURE ATLEAST EQUIVALENT TO THE BOILING POINT OF THE LIQUID OF SAID LIQUIDSOLUTION, (D) MAINTAINING THE COATED WEB WITHIN SAID PRESSURE AREA FOR APERIOD SUFFICIENT TO CAUSE SAID THERMOSETTABLE MINERAL COATING TO BECOMEPRESSURE-CAST ON SAID WEB, (E) SEPARATING SAID AT LEAST TWO MOVINGENDLESS BELTS TO RELEASE THE PRESSURE ON SAID WEB, (F) RECOVERING THERESULTING THERMOSET PRESSURE CASTCOATED WEB, AND (G) COOLING THESEPARATED SAID ONE MOVING ENDLESS METAL BELT TO A TEMPERATURE AT LEASTBELOW THE BOILING POINT OF SAID LIQUID PRIOR TO ITS CONTACT WITH SAIDPUDDLE.